The renaissance in the synthesis of small organic molecules with strong electron-accepting properties (i.e., organic semiconductors (OSCs)) during the past decade has been spurred by the global economic needs of (i) renewable sources of energy and (ii) more efficient and less expensive electronic devices that preferably include earth-abundant elements. The replacement of silicon- and metal-based electronic components with rationally-designed, finely-tuned organic molecules and molecular assemblies should lead to sustainable, lower-cost, lower-weight, flexible, yet robust solutions to many 21st century technological problems. Significant progress in this endeavor requires a deep understanding of the fundamental relationships between molecular structure, solid-state morphology, and electronic and other physicochemical properties of OSCs.
It has long been recognized that the presence of electron withdrawing groups (e.g., halogen atoms, cyanide, perfluoroalkyl (RF), or perfluoroaryl groups) in polycyclic aromatic hydrocarbons (PAHs) results in molecules with enhanced electron acceptor properties, better air stability, and improved solid-state charge-carrier mobilities. In particular, organic transistors made with the few available PAH acceptors bearing RF groups have long-term air-stability, which can be correlated with either low LUMO energies (ca. −4 eV; estimated by cyclic voltammetry), or with DFT-predicted gas-phase electron affinities (EAs) above 2.8 eV (believed to be the threshold value for n-channel-device air stability. The arsenal of synthetic methods used in the past to prepare PAHs with one or more RF substituents include (i) bottom-up design involving multi-step coupling reactions of smaller precursors already bearing an RF group (U.S. Pat. No. 7,390,901 (Yang et al.); Geib et al., J. Org. Chem. 2012, 77, 6107-6116), (ii) metal-catalyzed reactions in solution for modification of PAH intermediates that have reactive substituents such as Cl or Br atoms, intermediates that are themselves not commercially available (Schlosser, Angew. Chem. Int. Ed. 2006, 45, 5432-5446; Tomashenko and Grushin, Chem. Rev. 2011, 111, 4475-4521), and (iii) direct perfluoroalkylation of PAHs (Bravo at al., J. Org. Chem. 1997, 62, 7128-7136). However, the latter method has not been extensively studied, possibly because the low yields and poor regioselectivities reported were not encouraging (Tiers, J. Amer. Chem. Soc. 1960, 82, 5513-5513; b) Cowell and Tamborski, J. Fluorine Chem. 1981, 17, 345-356).
Accordingly, there is a need for new methods for the direct perfluoroalkylation of PAHs and related aromatic and heteroaromatic compounds. There is also a need for new small organic molecules with strong electron-accepting properties, for example, for use as organic semiconductors in electronic devices.